Gellan is a gum consisting of an exocellular, microbial polysaccharide, produced from Pseudomonas elodea cell lines, composed of repeating tetrasaccharide units with the following structure: EQU -3-.beta.-D-glcp-(1-4)-.beta.-D-glcpA-1-4)-.beta.-D-glcp-(1-4)-.alpha.-L-rh amp-(1-
wherein "glcp" designates glucose, "glcpA" designates glucuronic acid, and "rhamp" designates rhamnose.
In its natural form, gellan gum contains an O-acetyl group at position C(6) of the first glcp residue and an O-glyceric group at position C(2) of the same residue. Natural gellan gum forms viscous solutions that can form fragile and heat-sensitive gels upon the addition of a salt, in particular, with bivalent cation salts such as Ca.sup.2+ and Mg.sup.2+ salts.
Deacylation of natural gellan leads to an improved gelling agent, currently sold under the name of "Gelrite.RTM." for formulations in the alimentary field. In the presence of aqueous solutions of MgCl.sub.2 or CaCl.sub.2 (approximately 0.1% w/v salt and 0.8-1% w/v polysaccharide) Gelrite.RTM.0 forms highly resistant gels, that remain stable after autoclaving, are chemically inert and generally resistant to enzymatic digestion. Gellan is useful because of the ability of gellan chains to undergo a salt-inducted coil.fwdarw.double helix conformational change in water at room temperature. At higher ionic strengths, double helical sections form salt-stabilized partial aggregates which, eventually, constitute the "junction-zones" of the final aqueous gel state. The stability of the ordered double helical chain state as well as of the gel state depend on the nature of gellan counterions, since they influence the stability of the double-helix polysaccharide. It is to be expected that alteration of the charge density of the gellan chains by partial esterification of the carboxy groups may notably influence the polymer's properties in solution, producing derivatives with new gelling properties.
Auto-cross-linked esters of other polysaccharides are known (see European Patent Publication No. 0341745), but not auto-cross-linked esters of gellan. (Italian Patent Application No. PD91A000033 discloses outer esters of Gellan, but not auto-cross-linked esters of gellan).